Various electric devices which need calculations and control such as a central arithmetic unit (CPU) mounted on a small microcomputer, a personal computer, or a server have been constructed from a large number of transistors with advancement in function. A transistor is operated by electric power. The consumed electric power in operation tends to increase with the operation speed (clock rate). For example, a penetration current generated at the time of switching of a CMOS inverter increases with the increase in a clock rate. The penetration current flows out of a power supply into a grounding conductor. By the current change through the grounding conductor, an electromagnetic wave is emitted with the grounding wiring as an antenna. Larger is the amount of change of current, stronger is the radiated electromagnetic wave. The electromagnetic wave thus generated without intention is called a leakage electromagnetic wave.
Groups of transistors are mutually connected with the wiring which is stretched around the inside of an integrated circuit (IC). Metals, such as copper, and aluminum, are generally used as a material of wiring. When electrons pass through the wiring, an electric and a magnetic fields change around the wiring due to the variation of the amount of electrons with time passing through the wiring. The wiring is not necessarily closed within the IC. Some of information transmitted along the wiring (binary information corresponding to 0 or 1 in many cases) is transmitted from a small-outline integrated circuit forming a chip to the outside or transmitted from the outside. Here, the outside may be an exterior of a chip considered in the unit of chip, or an exterior of a module considered in the unit of functional module, and may not necessarily be the physical outside of an IC chip. This is because a technique such as a SoC (System on Chip) is used wherein plural functions are integrated into one chip such as a computer and a storage device. Another technique is also developed in which chips are not explicitly connected with wiring, but electrode pads are stuck together, thus making the boundary not always clear between the inside and the outside of an IC chip. However, it is still true that the exchange of information is performed through the wiring which connects between plural functional modules.
In a general logic circuit in which a signal is considered to be 1 when the voltage of the signal is equal to or larger than a certain threshold, and 0 when the voltage is less than the threshold, it is important that a voltage is maintained at a predetermined value between the transmitting and the receiving side of the signal. A voltage drop occurs as the distance of a line becomes long between the transmitting and the receiving side of a signal due to an increase in electrical resistance along the line. Accordingly, the voltage of the signal is raised to a high value at the transmitting side so that the signal may not decline below the threshold in the course of transmitting as the length of the wiring is long for transmitting the signal. The potential difference between the cases of transmitting 1 and 0 is clearly large. The larger the variation of the voltage in unit time is, the larger the variation of the electric field generated in the surroundings is. Similarly, since a change of current occurs at switching, a change of magnetic field is also generated. Thus, an electromagnetic wave is generated corresponding to the change of voltage. That is, transmitted information corresponding to the change of voltage can be indirectly known by observing the change of the magnetic and electric fields generated in the surroundings of wiring. In order to respond to the voltage drop along a wiring path, an amplifier, etc. are installed on the route and a method of raising the voltage, etc. are employed. Even in this case, it is the same that there exist correlation between the voltage variation and the transmitted information.
For a device which is miniaturized and consumes only a small power, variation of the magnetic and electric fields caused by the phenomena described above is small since such a device has a short wiring and needs low operation voltages. And the electromagnetic wave generated when information terminals such as a computer operate, is controlled so as to make the intensity small by following the standard of VCCI (Voluntary Control Council for Information Technology Equipment), etc. Even so, it is possible to intercept faint electromagnetic waves by using a highly efficient antenna and receiver.
The information about the operation condition of the information terminal represented by the data currently transmitted is included in the intercepted electromagnetic waves. Accordingly, if the intercepted information is analyzed appropriately, the information about the contents of keying of a keyboard, a display to a monitor, etc. can be read out. Since a monitor used for a personal computer, etc. is also constituted from transistors and operated as shown in FIG. 4, electromagnetic waves are generated from the monitor 401 corresponding to an operating state. Although the electromagnetic waves thus generate are generally weak, it can be intercepted by using a highly efficient antenna 402 and a highly efficient receiving set 403. By analyzing and displaying an interception result on a monitoring unit 404 for displaying an interception result, an image can be reproduced corresponding to the contents of a display of the monitor 401.
Besides the electromagnetic waves propagated through the air, there also exists an electromagnetic wave propagating through metallic parts of the power cable or various kinds of the connecting cables of a device. For example, the voltage of a grounding terminal is changed due to a voltage variation generated in transmitting a signal or a penetration current generated in switching of a transistor, and the voltage change of the terminal propagates through the power cable of the device. Since the electric signal propagating in the metal has a smaller attenuation in magnitude than in the air per unit distance and mixing of noise is less, the interception of the signal is attained at a distant place.
As a measure, as disclosed in JP-A-Hei6(1994)-209180, for preventing information from interception outside of the building is performed by shielding the wall of a building, and leakage of electromagnetic waves is confined inside of a device by shielding treatment for the housing of a device to absorb and intercept the electromagnetic radiation. Other measures are taken such as mounting a low pass filter to a signal cable to reduce a leakage of high frequency electromagnetic wave, and generating jamming wave to jam interception of leakage electromagnetic waves.